The present invention relates to electrical signal measuring devices and deals more particularly with an apparatus or circuit for simultaneously measuring and displaying the voltage, current and frequency for a selected phase in a polyphase alternating current system.
Apparatus for measuring voltage or current in polyphase systems generally include expensive and hard to read analog meters for indicating the magnitude of the measured voltage or current. Such measuring apparatus is often cumbersome to use and difficult to connect or wire to the involved system. Generally, a separate meter is required to display the voltage or current associated with each leg or phase of the system.
A single meter is sometimes used for indicating voltage or current by switching it to the desired phase to be measured. In such cases a rotary switch or other such electromechanical transferring device is usually used to selectively connect the meter to the appropriate leg of the system associated with the phase to be measured, and the switch or other device used must be capable of withstanding breakdown caused by the high voltages and currents generally associated with polyphase systems.
The high current in a polyphase alternating current system is usually reduced to a lower value by a current transformer to permit usage of a standard current meter connected to the transformer to indicate the value of a measured current. The transformer is connected so that its primary winding is in the circuit carrying the high current and its secondary winding is connected to current meter. Such a current transformer typically provides a current transformation from 500, 1,000 or 2,000 amperes to 5 amperes.
In the use of current transformer having a 5 ampere maximum applied secondary current and a standard 5 ampere current meter, for example, to measure the current flowing through the current transformer secondary winding, it is necessary in order for the meter to indicate the value of the current flowing in the primary circuit that the face of the meter be marked to reflect the transformation value of the current transformer to which the meter is connected. For example, a 500 ampere to 5 ampere transformer has a 100 to 1 transformation value and the face of a meter used with a transformer having such a transformation value is marked to indicate 100 amperes at the 1 ampere deflection point, 200 amperes at the 2 ampere deflection point and so forth. If a different transformer is used to measure a different maximum current flowing in the primary winding, the meter face must be either remarked or replaced by another face to reflect the transformation value of the different transformer. The remarking or replacing of a meter face is difficult and often time consuming and consequently, a number of meters having different faces are usually stocked for use with a number of different current transformers having different transformation values.
It is therefore a general aim of the present invention to provide apparatus for simultaneously measuring and displaying in digital readout format voltage, current and frequency values relative to a polyphase alternating current electrical system.
Another aim of the present invention to provide apparatus for measuring electrical signals in a polyphase alternating current system that can be used with a number of standard current transformers without requiring rewiring of the apparatus when transformers are changed.
A further aim of the present invention is to provide measuring apparatus that can selectively choose for measurement any one phase of the involved polyphase system.
A yet further aim of the present invention is to provide measuring apparatus that can be selectively conditioned to operate in a line-to-line or a line-to-neutral measuring mode.
A yet further aim of the present invention is to provide measuring apparatus that is easily read, inexpensive and easily manufactured using standard components.
The above and other features and advantages of the invention will be apparent from the following detailed description of a solid-state semiconductor implemented measuring apparatus, and from the claims, taken in conjunction with the accompanying drawings.